It is generally of interest in computed tomography (CT) examinations to keep both the amount of injected contrast agent and the radiation dose to a minimum so that the patient is not exposed to unnecessary stresses. However, this often cannot be achieved just like that. Thus, for example, when examining stroke patients using computed tomography, two further CT scans are generally performed with administration of contrast agent after a native CT scan.
Here, this is a dynamic perfusion measurement and a subsequent CT angiography (CTA). In the case of the dynamic perfusion measurement, use is made of an injection protocol with a high flow rate of at least 5 ml/s for the contrast agent in order to produce a short, sharp contrast agent bolus, which is a precondition for calculating perfusion parameters such as blood flow and blood volume.
In CT angiography, a contrast-agent injection protocol is used that produces a longer, wider contrast agent bolus. Here, a larger overall amount of contrast agent is used and the flow rate is lower. This leads to more regular enrichment of the contrast agent in the vessels and longer vessel sections being able to be illustrated in the angiography. The CT angiography is used to be able to determine the origin of a perfusion disorder. The two last-mentioned examinations lead to a significant amount of contrast agent injected into the patient and to a high radiation dose to which the patient is exposed.
Relatively new techniques in computed tomography, such as the so-called adaptive 4D spiral (A4DS) from Siemens allow perfusion measurements in the skull that are no longer limited by the width of the detector to a few layers, but which can be carried out in the entire skull volume with a cover of up to 10 cm. The measurement data from such a perfusion computed tomography scan can be used not only for calculating perfusion parameters, but also for generating dynamic CT angiographies, which illustrate the inflow and outflow of the contrast agent in the vessels. Such a dynamic angiography however cannot replace the conventional static CT angiography because in each time phase, illustrated by the dynamic angiography, only a short vessel section is enriched with contrast agent due to the short contrast agent bolus.
US 2009/0028409 A1 discloses a method for processing measurement data from perfusion computed tomography as per the preamble of patent claim 1. In this document, blood vessels are extracted by way of a thresholding technique.
US 2007/0016016 A1 describes inter alia a method for processing measurement data from perfusion computed tomography, in which an image is obtained from the data records from a dynamic CT angiography by way of MIP over the entire period of time or an averaged image is obtained by averaging. P. Montes and G. Lauritsch: “Noise Reduction by Temporal Estimation in Perfusion Computed Tomography”, in: IEEE Nuclear Science Symposium Conference Record, 2005, M11-372, pages 2747-2751, describe techniques for reducing noise in perfusion computed tomography, wherein one refinement carries out temporal smoothing in order to improve the signal-to-noise ratio. The smoothing is brought about for example by the use of a suitable low-pass filter.